The present application generally pertains to luminescent solar concentrators and more particularly to transparent luminescent solar concentrators for integrated solar windows.
Integrating solar-harvesting systems into the built environment is a transformative route to capture large areas of solar energy, lower effective solar cell installation costs, and improve building efficiency. Widespread adoption of solar-harvesting systems in a building envelope, however, is severely hampered by difficulties associated with mounting traditional solar modules on and around buildings due to cost, architectural impedance, and mostly importantly, aesthetics.
The concept of luminescent solar concentrators (“LSCs”) is well known, and with recent advances in phosphorescent and fluorescent luminophore efficiencies, LSC system efficiencies have increased to 7.1%. Although optical funneling of light limits the overall system conversion efficacy to less than ten percent (without LSC stacking), it can dramatically reduce the area of expensive solar cells needed, driving down the overall installed cost and increasing the ratio of electricity generation to solar cell surface area. Because of the high cost of glass and real-estate that factor into the module and the balance of systems costs, respectively, such LSCs have rarely been adopted in solar-farm practice despite the increasing performance and potential for low module costs. Furthermore, there has been demonstrated interest in utilizing LSCs as architectural windows. To date, however, these systems have been limited to absorption and emission (glow) in the visible part of spectrum, hindering widespread adoption of such devices in windows. In general, the purpose of windows is to provide natural lighting with a view; that is, most people prefer not to work behind strongly colored glass. A high level of untinted-transparency is therefore desirable for ubiquitous adoption.
The performance of LSCs can be understood by the component efficiencies: luminophore photoluminescence efficiency, solar spectrum absorption efficiency, waveguide (trapping) efficiency, solar cell efficiency, and transport (re-absorption) efficiency. The processes are schematically highlighted in FIG. 1 with a summary of the highest achieving systems to date with their corresponding dimensions in Table 1. The highest performance LSCs utilize phosphorescent organic molecules or blends of multiple fluorophores (such as quantum dots or organic dyes) that act to reduce reabsorption (Stokes shift) losses and enhance overall absorption efficiencies across the spectrum. The highest efficiencies reported (6-7%) have been for relatively small plates (<0.1 m2), since larger LSCs sizes suffer substantial reabsorption losses that limit efficiencies to <5%. A summary of key aspects of each loss mechanism is described below:
TABLE 1Highest preforming LSC demonstrations withcorresponding area dimensions:LSC SizeVisiblyChromophoreCell(m × m)t (mm)Effic. (%)ColoredRefDCMSi1.2 × 1  41.3Yes5CoumarinSi1.2 × 2  42.3Yes5Coumarin, RhodamineSi1.4 × 1.4303.2Yes6Eu(TTA)3(TPPO)2Si0.9 × 0.930.11No7CdSe/CdSGaAs1.4 × 1.4304.5Yes6CdSe/CdSSi0.05 × 0.0532.1Yes8Red305, CRS040GaAs0.05 × 0.0557.1Yes9BA241, BA856GalnP0.02 × 0.0236.7Yes10Pt(TPBP)CdTe0.1 × 0.114.1Yes11Rubrene, DCJTBCdTe0.1 × 0.114.7Yes11DCJTB, Pt(TPBP) Tand.CdTe0.1 × 0.116.1Yes11
Absorption Efficiency:
For LSCs with down-converting luminophore dyes, the absorption efficiency is necessarily lower than the absorption efficiency of the attached PV. For reference, Si solar cells have 50.3% absorption efficiency for the solar spectrum, shown in FIG. 2. Integrating the solar photon flux there is approximately 5.0% in the UV (300-435 nm), 21.5% in the visible (“VIS”) (435-670 nm), and 73.5% in the NIR (670-3000 nm).
PV Losses:
PV losses stem from the intrinsic thermodynamically-limited shape of the current-voltage curve. As the solar cell band gap decreases, voltage and fill-factor losses increase. Characteristic PV efficiencies illuminated by AM1.5G are shown in FIG. 3. Due to the monochromatic emission nature of the LSC, only single junction PVs can be considered around each individual LSC, defining the upper bound for the solar cell efficiency, ηPV, to that of the single-junction defined by Shockley-Quiesser.
Waveguide Efficiency:
Waveguiding relies on the principal of total internal reflection around a critical emission cone that results from the index variation between the waveguide and the waveguide cladding, in otherwords, air. Waveguide losses are typically 20% for waveguide substrates with an index of 1.5-1.7. Waveguide smoothness and optical transparency also play an important role as waveguides are scaled to >m2 areas.
PL Efficiency:
Typical quantum yields (“QY”) for down-converting chromophores now readily reach values greater than 50% for a variety of materials including organic phosphors, organic fluorophores, and colloidal quantum dots. Up-converters or anti-Stokes materials still have notably low luminescent efficiencies <5%, and typically, the quantum yields of these materials are notably limited at <1-2% QY.
Reabsorption Losses:
Light emitted by the luminophore in the waveguide must traverse the length of the waveguide before being reabsorbed by the dye or waveguide to reach the solar cell and produce power. These losses are dependent on the quantum yield of the dye, the overlap (or Stokes shift) of the dye emission-absorption, and the overall waveguide dimensions. It has been shown theoretically, that low QY massive Stoke-shift materials can outperform fluorophores with unity quantum yields over large LSC dimensions. That is, even for luminophores with 100% quantum yield, reabsorption losses can become dominant for luminophores with small Stokes-shift in large waveguides since each absorption/emission event leads to a reduction of photon flux through cone emission from the front of the waveguide that effectively act as scattering events.
It has long been recognized that LSCs are most limited by reabsorption losses, particularly for larger plate sizes. Indeed, much of the research with LSCs has focused on the reduction of these reabsorption losses through increasing Stokes shifts with organic phosphors, multiple dye optimization to artificially increase the Stokes-shift or resonance shifting, applicable only to neat-film dye layers less than several microns thick.
Previous efforts to construct transparent solar-harvesting architectures have focused on: (1) semi-transparent thin-film photovoltaics that typically have severe tinting or limited transmission or have an inherent tradeoff between efficiency and transparency, (2) LSCs incorporating colored chromophores that absorb or emit in the visible, or (3) optical systems using wavelength dependent optics that collect direct light only and reguire solar tracking. All of these approaches are severely limited in their potential for window applications due to aesthetic properties, bulkiness, or considerably limited transparency. These approaches suffer from an inherent tradeoff between power conversion efficiency (“PCE”) and visible transparency (“VT”), since both parameters cannot be simultaneously optimized in conventional devices. Architectural adoption is impeded further with typical organic PVs that have non-uniform absorption within the visible spectrum, resulting in poor color rendering index (“CRI”), high colored tinting and poor natural lighting quality. In contrast, it would be desirable to obtain visibly transparent, UV/NIR-selective LSCs to avoid aesthetic tradeoffs (low VT or CRI) that hinder architectural adoption and provide a clear route to large area scaling.
Various conventional devices employ a luminescent solar collector having luminescent agents dispersed throughout. Exemplary U.S. patent Nos. include: U.S. Pat. No. 4,155,371 entitled “Luminescent Solar Collector” which issued to Wohlmut et al. on May 22, 1979; U.S. Pat. No. 4,159,212 entitled “Luminescent Solar Collector” which issued to Yerkes on Jun. 26, 1979; U.S. Pat. No. 4,357,486 entitled “Luminescent Solar Collector” which issued to Blieden et al. on Nov. 2, 1982; 2009/0027872 entitled “Luminescent Object Comprising Aligned Polymers having a Specific Pretilt Angle” which published to Debije et al. on Jan. 29, 2009; and 2010/0288352 entitled “Integrated Solar Cell Nanoarray Layers and Light Concentrating Device” which published to Ji et al. on Nov. 18, 2010. All of these are incorporated by reference herein.